The use of fossil fuel in gas turbine engines results in the combustion products consisting of carbon dioxide, water vapor, oxides of nitrogen, carbon monoxide, unburned hydrocarbons, oxides of sulfur and particulates. Of these above products, carbon dioxide and water vapor are generally not considered objectionable, In most applications, governmental imposed regulations are further restricting the remainder of the constituents, mentioned above, emitted in the exhaust gases.
The majority of the products of combustion emitted in the exhaust can be controlled by design modifications, cleanup of exhaust gases and/or regulating the quality of fuel used. For example, particulates in the engine exhaust have been controlled either by design modifications to the combustors and fuel injectors or by removing them by traps and filters. Sulfur oxides are normally controlled by the selection of fuels that are low in total sulfur. This leaves nitrogen oxides, carbon monoxide and unburned hydrocarbons as the emissions of primary concern in the exhaust gases emitted from the gas turbine engine.
One of the approaches to reducing emissions utilizes the lean premix combustion concept. In this approach the fuel and air are uniformly premixed before they enter the combustion zone of a combustor and the fuel/air ratio is controlled so that there is a relative excess of air as compared to the stoichiometric fuel/air ratio. In the lean premix combustion concept it is possible to control emissions by controlling the fuel/air ratio within the combustor, the other variables being primarily dependent variables.
The principal mechanism for the formation of oxides of nitrogen involves the direct oxidation of atmospheric nitrogen and oxygen. The rate of formation of oxides of nitrogen by this mechanism depends mostly upon the flame temperature and, to some degree, upon the concentration of the reactants. Consequently, a small reduction in flame temperature can result in a large reduction in the nitrogen oxides.
If the flame temperature is reduced too far, significant levels of carbon monoxide are left in the exhaust gas because the oxidation rate of carbon monoxide to carbon dioxide slows down and eventually stops. The carbon monoxide is present because it is formed during the reaction of the hydrocarbon gases with the air.
Some examples of mechanical devices used to control flame temperature are variable geometry injectors or other types of systems to influence the ratio of fuel and air within the combustor. In controlling such devices, control systems must be formulated. For example, as shown in U.S. Pat. No. 5,309,709 issued on May 10, 1994 to Philip J. Cederwall et al., such a system is disclosed. In the disclosed control system, the air/fuel ratio is controlled. For example, the system includes a manifold in which the quantity of compressor air directed thereto is controlled. The air flow from the manifold is directed to a plurality of injectors and hence the air/fuel ratio within a combustor is controlled or regulated. A throttling mechanism moves between an open position and a closed position varying the flow rate of compressed air to the manifold and consequently, to the combustor. Excess air not used for cooling or combustion is dumped to the atmosphere.
The system defined above uses measured operating level, such as temperature, to correlate what is assumed to be the level of emission being emitted from the gas turbine engine. The actual measured emission is only accomplished by external systems which are usually performed at random specified intervals. The use of actual measured emission systems is complex, time consuming and has a slow response. Normally, actual emission measurement systems include taking a sample or samples, transporting the sample to an analyzer, having the sample analyzed, assessing the results of the analysis and changing parameters of the engine operation depending on the results of the analysis. Such actual emission measurement systems are expensive, result in a slow turn around time and are complex to operate.
The above systems used therewith are examples of attempts to control the emissions of carbon monoxide and oxides of nitrogen.
The present invention is directed to overcome one or more of the problems as set forth above.